The U.S. Justice Department on May 17 appointed former FBI Director Robert Mueller as special counsel to investigate possible collusion between Russia and President Donald Trump's 2016 campaign team.

The move, which was welcomed by both Democrats and Republicans in Congress, came amid rising demands for an independent probe of alleged Russian efforts to sway the outcome of November's presidential election in favor of Trump and against Democrat Hillary Clinton.

U.S. Deputy Attorney General Rod Rosenstein said he decided to appoint a special counsel not because he believes "crimes have been committed or that any prosecution is warranted," but because he believes "a special counsel is necessary in order for the American people to have full confidence in the outcome" of the investigation.

"The public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command," Rosenstein said.

A special counsel is considered independent because he has the authority to conduct an investigation with a staff of his own choosing and without any requirement to consult with or inform the Justice Department and White House about the course of the investigation. The special counsel also is authorized to prosecute any suspected crimes unearthed by the investigation.

Suspicions that the White House may have tried to influence the Justice Department's own investigation of the Russian matter, which the FBI began last year, arose when Trump fired FBI Director James Comey a week ago.

Further questions arose on May 16 when reports surfaced that Trump had asked Comey during his first weeks in office to end an investigation into Russia's ties with former national security adviser Michael Flynn.

Comey wrote about Trump's request as well as other conversations he had with Trump in memos that were widely reported by media on May 16. The White House denied Comey's account of his conversation with Trump about Flynn.

Several congressional leaders said Trump's request to quash the Flynn investigation may have amounted to obstruction of justice -- a criminal offense in the United States.

By the evening of May 17, three congressional committees had demanded to see Comey's memos and were seeking to have Comey testify about his discussions with Trump.

After the special counsel was named, Trump issued a statement late on May 16 saying that, once again, there were no questionable ties between Russia and his campaign.

"A thorough investigation will confirm what we already know: There was no collusion between my campaign and any foreign entity," he said. "I look forward to this matter concluding quickly."

Earlier in the day, Trump had complained about media coverage of the Russian matter, saying that "no politician in history -- and I say this with great surety -- has been treated worse or more unfairly."

Congressional leaders praised the decision to appoint a special counsel -- something Democrats have been urging since last year, with many Republicans joining the chorus in recent days.

Senate Democratic Leader Charles Schumer said Mueller, who served as FBI director from 2001 to 2013 during both Republican and Democratic administrations, was the right choice for the job.

“A special counsel is very much needed in this situation and Deputy Attorney General Rosenstein has done the right thing," Schumer said.

Nancy Pelosi, the Democratic leader of the House of Representatives, called Mueller "a respected public servant of the highest integrity."

She added that "a special prosecutor is the first step, but it cannot be the last. Director Mueller will still be in the chain of command under the Trump-appointed leadership of the Justice Department."

"I believe Mueller will be independent, he will be thorough, and he will be fair, and he's not going to be easily swayed," said Elijah Cummings, the top Democrat on the House Oversight Committee, which is also investigating the Russian matter.

Republican Jason Chaffez, chairman of the House oversight panel, said Mueller was a "great selection. Impeccable credentials. Should be widely accepted."

Senate Intelligence Committee Chairman Richard Burr, whose committee is also investigating the Russian matter, said appointing a special counsel was a "good decision" that "assures the American people that there's no undue influence" in the Russia investigation.

U.S. intelligence agencies concluded earlier this year that Russia interfered in the election, and the FBI's investigation was building on those findings. Moscow has repeatedly denied any interference.